1. Field of the Invention
The present invention relates to a vehicle brake system provided with a hydraulic brake device and a regenerative brake device. More particularly, it relates to a brake control device for cooperatively controlling the hydraulic brake device and the regenerative brake device.
2. Discussion of the Related Art
In hybrid vehicles which are provided with an engine and a generator motor as travelling drive sources, it has become widespread to heighten the fuel efficiency by regenerating motion energy to electric energy by the generator motor and storing the electric energy at the time of a braking operation. In this sense, the generator motor is regarded as a regenerative brake device that applies the regenerative brake force to driving wheels. The regenerative brake device alone is unable to generate a sufficient brake force and thus, is usually used in combination with a conventional hydraulic brake device which operates under pressurized oil. Therefore, a cooperative control is required for the hydraulic brake device and the regenerative brake device, and there have been proposed various cooperative control technologies like that described in US 2007/0272457 A1 (equivalent of JP2007-308005 A).
A vehicle disclosed in the United State publication is provided with a combustion engine, an electric motor, battery means, fluid-operated brake means (hydraulic brake device), demand brake force setting means, and brake control means. The fluid-operated brake means is able to output a brake force based on a manipulation pressure (base hydraulic pressure), corresponding to the driver's manipulation and a negative pressure in the combustion engine, and an additional pressure (controlled hydraulic pressure) given by pressurizing means. Further, when a brake demand manipulation is performed, the brake control means executes a control to compare a sum of a regenerative brake force by the electric motor and a manipulation brake force corresponding to the manipulation pressure with a demand brake force and judges the necessity of a brake force depending on the additional pressure. The control makes it possible that even when the negative pressure in the combustion engine goes down, a demand brake force is acquired correctly by suppressing an uncomfortable feeling which is liable to be felt by the driver.
Further, although the hydraulic brake device usually operates in response to the braking manipulation by the driver and, in addition to this function, is often to have a function of automatically adjusting the brake force to be increased or reduced. Such an automatic brake control function is realized in a combination of an electronic control device, incorporating a computer and being operable by software, and sensors for acquiring various information such as braking manipulation amount, wheel speeds and the like. For example, in an active cruise control (ACC) function, a following distance (i.e., a distance to a vehicle ahead) is kept to be longer than a predetermined value by generating a brake force in dependence on the situation where a braking manipulation is not performed or the amount of the braking manipulation is insufficient though a detected following distance decreases. In a brake assist (BA) function, it is discriminated based on a braking manipulation amount and a manipulation speed whether or not a braking manipulation is an urgent braking manipulation, and an additional brake force is added to the brake force corresponding to the braking manipulation force. Further, in an antilock brake system (ABS) function, when a wheel is locked at the time of an urgent braking manipulation, the hydraulic pressure in the hydraulic brake device is automatically adjusted to regulate the brake force thereby to suppress the slipping of the wheel. Those belonging to this category are a traction control (TRC) function that controls the driving force to be effectively exerted on the road surface by adding a brake force when the slipping amounts of the driving wheel are large, and an electronic stability control (ESC) function that keeps the stability in travelling by regulating the braking amounts of respective wheels during the travelling.
In systems provided with a hydraulic brake device and a regenerative brake device as typically described in the aforementioned United State publication, when a braking manipulation is done, the regenerative brake device is additionally operated whereas, during an automatic brake control function such as the ACC function, the BA function or the like, the brake force is regulated only by the hydraulic brake device with the regenerative brake device held out of operation. That is, a stage that the regenerative brake function by a generator motor can be utilized takes place unless the driving wheels are being driven when the brake force is generated by the automatic brake control function. However, during the automatic brake control function, the regenerative brake function has heretofore not been utilized for the reason that the utilization of the regenerative brake function at that stage complicates the distribution of brake forces to respective wheels, and the like. As a result, the hydraulic brake device only has been used even at the stage that the regenerative brake device can be inherently utilized, and thus, the opportunity to enhance the efficiency in regeneration has been lost.